This invention relates to guaranteeing stable lighting of a discharge lamp and preventing degradation and short life of the discharge lamp in a discharge lamp lighting circuit adapted to perform DC lighting over a predetermined time. The lamp lighting circuit temporarily defines the lighting frequency of the discharge lamp as low frequency just after the discharge lamp is lighted.
The configuration of a lighting circuit of a discharge lamp, such as a metal halide lamp, including a DC power supply circuit, a DC-AC conversion circuit, and a starter circuit is known. For example, in the configuration wherein a DC-DC converter is used as a DC power supply circuit, and a full-bridge type circuit comprising two pairs of semiconductor switch elements for performing switching control, and a driver circuit thereof are used for a DC-AC conversion circuit, the positive-polarity (or negative-polarity) voltage output by the DC-DC converter is converted into rectangular-wave voltage in the full-bridge type circuit, then this voltage is supplied to a discharge lamp.
In order to light a discharge lamp more reliably at the starting time of the discharge lamp, preferably a period of temporarily supplying low-frequency voltage (so-called DC lighting period) is provided and alternating voltage of a stipulated frequency is supplied after the discharge lamp is lighted, rather than abruptly supplying voltage of a stipulated frequency just after the discharge lamp is lighted. To do this, a predetermined time is set as the duration of the period by a timer circuit or the like.
However, the state at the lighting time of the discharge lamp varies from starting the discharge lamp in a cold state to starting the discharge lamp in a warm state, and if the duration of the DC lighting period is defined evenly, variations in the start properties of discharge lamps occur or an excessive thermal stress is placed on the electrode of a discharge lamp; this is a problem.
It would therefore be desirable to guarantee stable lighting of a discharge lamp and prevent degradation and short life of the discharge lamp in a discharge lamp lighting circuit.
Provided is a discharge lamp lighting circuit comprising a DC power supply circuit for outputting a DC voltage, a DC-AC conversion circuit for converting the output voltage of the DC power supply circuit into an AC voltage and then supplying the AC voltage to a discharge lamp, and a current detection circuit for detecting an electric current flowing into the discharge lamp. Just after the discharge lamp is lighted, the output frequency of the DC-AC conversion circuit is temporarily changed for defining the lighting frequency of the discharge lamp as a low frequency, thereby performing DC lighting over a predetermined time. The duration of the DC lighting is determined by the time it takes for the product of the time and the value of current flowing into the discharge lamp to equal a predetermined value.
Therefore, according to the invention, the duration of the DC lighting is determined by the time it takes for the product of the time and the value of current flowing into the discharge lamp to equal the predetermined value. For example, when the current value of the discharge lamp is large, then the duration of the DC lighting is shortened; when the current value is small, then the duration is prolonged, so that the stability of lighting can be guaranteed in response to the state of the discharge lamp. Such operation prevents degradation and short life of the discharge lamp.